If you’re a Mozilla Firefox user and do any kind of Web Design, then the folks at www.designmeme.com have a list of 5 Firefox extensions that you should drop into your everyday toolbox. While I mostly agree with the list, I would definitely add the X-Ray Firefox extension to the list; The X-Ray extension let’s you see the tags on a page without viewing the sourcecode.
Top 5 Firefox Extensions for Web Designers
X-Ray Extension for Firefox